Version 1.2.1 - 28 Jul 2012
Allow for old JSON settings with only partial settings.
@Crewmom - I've released a new version that should address your issue. Please confirm that it works for you--assuming that it does.
(It's a corner case condition where you have an older set of saved configuration for the plugin that's not complete.)
As usual, there may be a delay before calibre notices the new version. You can get the latest by going in and telling calibre to install it again.